1. Field of the Invention
The present invention generally relates to the on-line detection of characteristics of sheet materials and, more particularly, to spectrometric detection of the characteristics of sheet materials.
2. State of the Art
Although various properties of sheet materials can be detected by laboratory tests, such tests have several inherent drawbacks. For example, one shortcoming of laboratory tests is that they require substantial time for sample acquisition and analysis. Another drawback of laboratory tests is that the samples obtained for testing may not accurately represent sheet material that has been produced.
To overcome the shortcomings of laboratory tests of sheet materials, devices have been proposed for detecting sheet properties "on-line," i.e., while a sheet-making machine is operating. On-line sensor devices for paper materials, for instance, can detect sheet properties such as basis weight, dry basis weight, moisture content, and thickness. It is well known to operate on-line sensor devices so that they periodically traverse, or scan, a traveling web of the sheet material. Normally, scanning is done in the cross direction; i.e., in the direction perpendicular to the direction of sheet travel. Depending upon the sheet-making operation, cross-directional distances can range up to about 400 inches or more.
Although a wide variety of sensor devices have been used for on-line scanning, there have been difficulties in attempts to use on-line spectrometers based upon interferometers. In part, the difficulties reflect the fact that interferometers inherently require precise positional relationships to be maintained to produce meaningful measurements. Also, the difficulties reflect the fact that interferometers normally are designed for laboratory use, not for use in manufacturing environments where there may be substantial vibrations from heavy an high-speed rotating machinery.